The present invention relates generally to the electrical, electronic and computer arts, and, more particularly, to capacitive touchscreens.
Capacitive touchscreens are ubiquitous interfaces for interacting with many electronic devices, the smartphone being a common example. To facilitate the entering of commands or the reading of a user's fingerprint, a capacitive touchscreen includes an array of pixel circuits, each pixel circuit sensing via capacitive coupling the proximal presence of a user, such as for example the user's finger or thumb. For some types of capacitive touchscreens, each pixel circuit comprises a capacitor plate, where the proximal presence of a user forms the complementary capacitor plate, thereby loading the pixel circuit with what is termed a touch capacitance. (The terms “capacitance” and “capacitor” are at times used interchangeably, it being understood that capacitance is an attribute of a capacitor.) A pixel circuit is also loaded by parasitic capacitance, but in practice the parasitic capacitance should be small compared to the touch capacitance.
To form a map (image) of the pixel circuit locations on a touchscreen that are loaded by a touch capacitance, for example for the detection of a fingerprint, a charge operation is performed on each pixel circuit followed by a read operation to determine those pixel circuits that are loaded by a touch capacitance. The pixel circuits are discharged (i.e., the effective parasitic and touch capacitors are discharged), and the process begins again to form another map of the touch capacitance locations. The time for performing the cycle of charge, read, and discharge operations can be referred to as the frame time. It is desirable for the frame time to be relatively small in value so that touch capacitance locations are accurately determined. It is also desirable to reduce power consumption by operating the touchscreen at a relatively small operating voltage, so that relatively small charges are involved in charging and discharging the touch capacitance.